Dusky sharks are commonly found in tropical coastal waters, but have been spotted in temperate environments when migrating. They descend to 400 meters at times. The depths that they inhabit depend on the environment they are in at the time. When swimming near the coast, they tend to stay at depths ranging from 50 to 100 meters. However, when inhabiting southern gulf waters they tend to stay at depths of 20 to 50 meters. (Hoffmayer, et al., 2014; Musick, et al., 2009; Rogers, et al., 2013)
Adult females typically range from 76 to 287 cm in length, whereas males are slightly smaller, having a range from 74 to 276 cm long. Length at sexual maturity is reported to be 235 cm (range 220-300 cm). The largest male dusky shark was 420 cm long. Adults weigh an average of 180 kilograms, but pregnant females have been known to weigh as much as 450 kilograms. The smallest adults weigh 160 kg. At birth, sharks are 48 to 100 centimeters in length.
As their name suggests, dusky sharks have dusky tips on their fins that are not very prominent. Their undersides are white and their snouts are rounded. Their pectoral fin is in the shape of a sickle. They are known to have the strongest bite of any shark species, being able to produce 60 kilograms of force when their jaws are fully closed. Adding to this bite force, their teeth are sharp, in the shape of a triangle, and serrated on both exposed sides. ("Appendix 1 Frequently asked questions about sharks", 2001; Carpenter and Valdestamon, 2017; Ebert, et al., 2013; Musick, et al., 2009; Natanson, et al., 2014)
The embryos at full term for a dusky shark can range from 48-100 cm (more typically 70-100 cm). This is a viviparous species with a yolk sac intially providing nourishment directly to the 3-14 young as they develop. When the yolk sac runs out of nutrients, the placenta then feeds the young. The female gestates the embryos for 16-22 months. Pups are born with working jaws and teeth and are able to hunt and kill from the time of birth. Like all other sharks, dusky sharks have indeterminate growth, meaning they continuously grow throughout their lifetime. Although more development data could and should have been collected during the era in which they were overfished, they were not. Currently, with smaller populations, very little research on development and other reproductive metrics is being conducted. (Carpenter and Valdestamon, 2017; Castro and Peebles, 2010; Ebert, et al., 2013; Hall, 2005; Hussey, et al., 2010; Romine, et al., 2009)
Male sharks have been known to mate with multiple females. Other than this there is no known information about the mating behavior of the dusky shark. (Ebert, et al., 2013; Hussey, et al., 2010; Musick, et al., 2009; Romine, et al., 2009)
Female dusky sharks are viviparous, giving birth to live young. They have a gestation period that typically lasts from 16 to 22 months. Potentially taking up to 2 years for their gestation period, they have to wait a year before being able to bear pups again, making their overall reproduction cycle 3 years long. They have a litter size ranging from 3-14 pups in one litter, with each pup ranging from 70 to 100 centimeters in length (or as small as 48 cm). They give nourishment to the pups by utilizing a yolk-sac placenta. Late term females lack a large yolky ova; this is one reason why they have to wait a year to mate again (Romine, 2009). Males tend to reach sexual maturity at the age of 19 years old, with a range from 17 to 24 years old, whereas females tend to reach sexual maturity at the age of 21 years old, with a range of 17 to 24 years old as well. There is no information about the mass of the pups when they are born. (Carpenter and Valdestamon, 2017; Ebert, et al., 2013; Hussey, et al., 2010; Musick, et al., 2009; Romine, et al., 2009)
Hussey (2010) reported that the pups are born with an oversized liver that accounts for 20 percent of the pups mass at birth. The liver contains stored energy for the pups to use within the first months of their lives. After the stored energy is used up, they are able to hunt and live freely. Ebert (2010) stated that immediately after birth, the mother moves away from the pups to avoid cannibalism. Ebert (2010) also suggested that the pups remain in a group shortly after birth. Hussey (2010) suggests that although they do not stay with the mother, the pups are not independent from their mothers' resource allocation after birth due to the excess nutrients within their liver that they are given in the womb.
Paternal investment is non-existent for the dusky shark. After impregnation, the father has nothing to do with the pups. (Ebert, et al., 2013; Hussey, et al., 2010; Musick, et al., 2009; Romine, et al., 2009)
Carpenter and Valdestamon (2017) report the oldest dusky shark was 40, but it's suspected the maximum age they can reach is mid-40s to 50s. The information does not specify whether this is for captive sharks or wild sharks. Compagno (1984) reports in the FAO Species Catalogue that younger dusky sharks fare well in captivity, but maximum lifespan has not been reported in captivity. ("FAO Species Catalogue, Vol.4. Sharks of the world An annotated and illustrated catalogue of shark species known to date Part 2. Carcharhiniformes", 1984; Carpenter and Valdestamon, 2017; Musick, et al., 2009)
Dusky sharks migrate from location to location depending on the temperature and season at the time, trying to stay in the more tropical areas during all times of the year. The females move closer to the shore when they are about to give birth, then move away from shore afterwards. The pups typically form schools that are sometimes divided up between males and females and hunt together. As they get older, they move away from the shore to more open waters but still moderately close to the shore. They have been known to follow ships that are leaving offshore areas at times. They have also been known to be extremely aggressive. There have been multiple reports of them attacking humans (Ebert, 2013). (Ebert, et al., 2013)
Dusky sharks don't have a reported home range, nor do they defend a territory. (Ebert, et al., 2013)
All sharks have sense organs that help them communicate. These organs monitor both the outside and inside environments at all times. Some of these organs are spread around the entirety of the sharks' bodies and transmit nerve impulses. These nerve impulses are sent to two different areas in the sharks' bodies, their brain and their spinal cord. When there are signals sent to the brain, they comprehend them as just sensations. When signals are sent to the spinal cord, it results in a reflex reaction. They also have another system called the neuromast system. This systems organs consist of lateral lines, cephalic canals, ampullae of Lorenzini, and pit organs. Neuromasts are all over the body as well as on the surface of the skin and in sunken canals that are beneath the skin. These organs help detect a wide variety of sensations and assist the sharks with the sensations of their surroundings. They can detect water movement and use it to orient their movement, detect sound waves, detects potential threats, and locate food. Some of the neuromast system uses electroreceptors that, when electric pulses are fired off, an electric field is transmitted all around the shark. When the electric pulses cease, the field fades away. (Linzey, 2012)
Dusky sharks are generalist carnivores, more typically feeding close to the marine substrate. When they are young, they tend to prey on smaller organisms such as sardines (Sardina pilchardus) and small squid (Teuthida). As they grow and become adults, they start to hunt for larger prey such as groupers (Epinephelus striatus, Epinephelus polyphekadion, Epinephelus flavocaeruleus). Dusky sharks have been known to eat skates, rays, sea turtles, other sharks as well. Invertebrate materials include gastropods, crustaceans, and sea stars. They can sometimes mistakenly consume human trash. In few stomachs, bottlenose dolphins Tursiops have been found. However, it's thought that the marine mammals are consumed already dead, as carrion. Dicken et al. (2015) do report one case of dusky sharks attacking a calf humpback whale (Megaptera novaeangliae). ("FAO Species Catalogue, Vol.4. Sharks of the world An annotated and illustrated catalogue of shark species known to date Part 2. Carcharhiniformes", 1984; Carpenter and Valdestamon, 2017; Dicken, et al., 2015; Musick, et al., 2009)
Adult dusky sharks have very few predators that hunt them. When they are born, they have a risk of being prey to larger sharks in the area, but as they grow this threat disappears. The only predator of adults is humans, Homo sapeins, who catch them in fishing efforts and seek them out for their fins. (Carpenter and Valdestamon, 2017; Ebert, et al., 2013; Musick, et al., 2009)
The dusky shark is a generalist, preying on a wide variety of animals. They are an apex predator in their system. Some of the parasites that inhabit these sharks are tapeworms Anthobothrium laciniatum, and the parasitic monogenean (a flatworm), Dermophthirius carcharhini. (Bullard, et al., 2004; Ebert, et al., 2013; Musick, et al., 2009; Ruhnke and Caira, 2009)
Dusky sharks are very beneficial to humans in the food industry. Their meat is valued at a high price and their fins are valued at an even higher price. The fins are used in a soup called shark fin soup as well as the skin being used for leather. According Musick et al. (2009), the oil from the livers can be used to make an assortment of vitamins. They are also the subject to ecotourism in many areas because of their tendencies to live near coastal waters. (Ebert, et al., 2013; Musick, et al., 2009)
According to the IUCN Red List, dusky sharks are listed as "Vulnerable." Ebert et al. (2013) list them as endangered in the northwest and central Atlantic ocean. The US Federal List, CITES, and the State of Michigan List all state there is no special status involved with this species.
This is a severely overfished shark, caught as by-catch in tuna nets. Their fins, livers, and hides are highly sought-after by humans. Musick et al. (2009) reported that 144,000 and 767,000 dusky shark fins are traded each year. With fishermen seeking out juveniles in many portions of their range, and fishing tournaments (e.g., in Florida) purposely catching and keeping them, the populations have declined precipitously. Given that it takes these sharks nearly 20 years to reach breeding status, these sharks are imperiled.
Dusky sharks are now illegal to fish or kill in the United States. The ban on fishing of these sharks in the U.S. Atlantic is already seeing populations improve incrementally. In other areas, the ban hasn't worked; in 2003, despite the ban, over 2000 dusky sharks were harvested. Other countries, as well as international conservation efforts, have mentioned monitoring of these and other sharks - but implementation is slow. The long-term prognosis for this species is unclear. ("FAO Species Catalogue, Vol.4. Sharks of the world An annotated and illustrated catalogue of shark species known to date Part 2. Carcharhiniformes", 1984; Carpenter and Valdestamon, 2017; Ebert, et al., 2013; Musick, et al., 2009)
Justin Alouf (author), Radford University, Alex Atwood (editor), Radford University, Karen Powers (editor), Radford University, Joshua Turner (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
flesh of dead animals.
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease
humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
uses electric signals to communicate
parental care is carried out by females
A substance that provides both nutrients and energy to a living thing.
Animals with indeterminate growth continue to grow throughout their lives.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
makes seasonal movements between breeding and wintering grounds
eats mollusks, members of Phylum Mollusca
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.
An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).
an animal that mainly eats fish
having more than one female as a mate at one time
mainly lives in oceans, seas, or other bodies of salt water.
breeding is confined to a particular season
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
movements of a hard surface that are produced by animals as signals to others
uses sight to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
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